IC parts used in the high frequency circuit portion of a cellular phone or the like in the known art include MMICs (monolithic microwave integrated circuits), which are described in "UHF Band High Efficiency FET Amplifier for Mobile Communications" published in Mitsubishi Electric Technical Report, Vol. 67, No. 11, 1993. The MMICs in the known art include three types.
The first type of MMIC is formed by constituting the active circuit portion comprising a plurality of active elements in the form of an IC chip and by forming the circuit portion constituted of a passive element on the surface of the IC chip package through thick film printing or the like. With the first type, since the passive circuit which only occupies the pattern area, is separated from the GaAs (gallium--arsenic) substrate and only the active circuit constituted of active elements such as transistors can be formed on the GaAs substrate which can be miniaturized, thereby achieving a reduction in the production cost of the IC part as a whole.
Next, with the second type of MMIC, the passive circuit portion constituted of passive elements such as filter portions is integrated on the IC chip substrate constituted of active elements into a single chip. With the second type, by forming the passive circuit portion on the GaAs substrate where the active circuit constituted of active elements such as a plurality of transistors is formed, the passive circuit and the active circuit can be manufactured at the same time through the same process, thereby achieving an extremely high degree of mass productivity. Moreover, from the viewpoint of manufacturers of semiconductor parts, the second type provides potential for achieving a reduction in the cost of the entire IC part since it does not require any ceramic chip carrier, which normally must be purchased.
The third type of MMIC is constituted by forming the passive circuit portion and the active circuit portion on separate IC substrates although these IC substrates are of the same type. Normally, in a high frequency band exceeding several GHz to several tens of GHz, the active circuit portion is formed by using a compound semiconductor constituted of GaAs or the like. Furthermore, a GaAs substrate, with its specific resistance at 10.sup.8 .OMEGA.cm or more, provides superior insulation compared to Si, whose specific resistance is approximately 2.3.times.10.sup.5 .OMEGA.cm. Thus, a great deal of interest is focused on the advantage that a passive element such as a coil that can be used in the high frequency band may be formed on a GaAs substrate, and currently, development of high frequency MMIC circuits by employing a GaAs substrate is underway.
With the third type, since the active circuit portion and the passive circuit portion can be manufactured through the same process and the passive circuit portion is provided on a separate chip, it is possible to limit the portion that requires changes in design (impedance, etc.) in correspondence to the frequency band that is used. Because of this, by commonly using IC chips for the active circuit portion and by simply changing the IC chip for the passive circuit portion, it becomes possible to produce a series of IC parts in correspondence to various frequency bands. In particular, a GaAs substrate with a special additive included may be employed in order to draw out certain transistor characteristics in the high frequency band, and such a GaAs substrate tends to be a great deal more expensive than a normal GaAs substrate. Thus, by using different GaAs substrates for the active circuit portion, which is constituted of active elements requiring the semiconductor characteristics in the high frequency band and for the passive circuit portion constituted of passive elements that do not require the semiconductor characteristics, IC parts can be manufactured at a lower cost.
However, in the case of the first type of MMIC, the ceramic IC package in which the active electronic parts are mounted, is a component that must be purchased as far as a manufacturer of semiconductor parts is concerned. Thus, IC parts employing ceramic IC packages tend to be more expensive than IC parts with regular resin mold packages.
In addition, in the case of the second type of MMIC, since it is necessary to design the IC by adding an impedance matching circuit for each frequency to be used, its versatility as a part is poor.
Furthermore, with the second and third type of MMIC, while a GaAs substrate required for constituting the MMIC provides superior insulation, it is still extremely expensive, and it is difficult, therefore, to reduce the product cost.
Moreover, while, with the second and third types of MMIC, it is desirable to employ a conductor such as silver, copper or the like with a low specific resistance for the pattern of the passive elements formed on the MMIC, since these conductors react with the GaAs substrate, it is extremely difficult to use them in practice.
In addition, while the pattern of the passive elements is formed on a GaAs substrate in both the second type and the third type of MMIC, it is necessary to implement grounding for the ground electrodes frequently in the passive circuit. Normally, in the structure of the substrate for constituting the passive circuit, the wiring layer on the substrate assumes a multilayer structure, a planar pattern of the ground electrodes is set in the lower layer and a signal line is set at a layer above it. If there is a node that requires grounding in the passive circuit (an electrode pattern within the circuit), the node is grounded to the ground electrode via a through--hole electrode. In order to achieve this substrate structure with a GaAs substrate, a method whereby a ground electrode is formed on the opposite side from the side where the circuit elements on the GaAs substrate are formed may be considered as a first option. Also a method may be considered whereby a ground electrode is first constituted on a surface where the elements are formed, next, an inorganic insulating layer is formed on the ground electrode and then a signal electrode is formed on the inorganic insulating layer as a second option.
However, with the first method, it is necessary to form holes passing through both the front and rear surfaces of the GaAs substrate being used, these holes, which are extremely fine, must be formed in great numbers and the internal surfaces of these holes must be made electrically continuous by conductors, all of which are practically impossible to achieve.
In addition, in the second method, the inorganic insulating layer formed between the ground electrode and the signal electrode is normally formed through vapor phase epitaxy since a high temperature process must be employed in the semiconductor manufacturing process. However, the inorganic insulating layer will achieve a thickness of only several microns through vapor phase epitaxy. This will result in a reduced line impedance in the signal line formed on the inorganic insulating layer, which, in turn, makes the circuit design extremely difficult. The line impedance in the high frequency band, in particular, will be extremely low. As a means for avoiding a reduction in impedance, the width of the signal line may be set at an extremely small value, but since the conductors are constituted of a thin film and, therefore, their thickness is at approximately several microns, if the width of the signal line is set at an extremely small value, the high frequency resistance will further increase, resulting in an increase in signal insertion loss.
Next, for high frequency circuits handling 100 MHz or more, various surface mounted high frequency passive electronic parts, which include passive circuits only, have been proposed and put into practical use. For instance, Japanese Unexamined Patent Publication No. 330136/1996, Japanese Unexamined Patent Publication No. 330154/1996 and Japanese Unexamined Patent Publication No. 330169/1996 disclose high frequency coils. To achieve the high frequency coils disclosed in these prior art publications, a spiral coil electrode pattern is formed on a ceramic substrate and the two ends of the coil electrode pattern are drawn out to the end portions of the substrate facing opposite each other to constitute a chip coil. As a means for coil electrode formation, thick film printing technologies, wet plating technologies, thin film technologies and the like are employed.
In the method employing thick film printing, since production can be carried out requiring only simple production facilities compared to facilities required for semiconductor production, an advantage is achieved in that production cost can be reduced. However, since the patterns are formed through screen printing, it is difficult to form extremely fine lines of 100 microns or less. Moreover, since smudging and blurring of the conductors tends to occur during conductor pattern printing, inconsistencies in the conductor width and the conductor film thickness may easily result. These negative factors ultimately result in problems in that, in the case of a high frequency coil, for instance, there is a limit to how fine the pattern can be made and there is also a limit to the extent of miniaturization that can be achieved and in that the inductance value tends to be inconsistent.
In the case of a method employing wet plating or the like, it is difficult to control the concentration of the plating solution in the plating bath at a constant level. In addition, since a processing facility for processing the waste liquid of the plating solution is required, the method tends to require large scale production facilities. Thus, production costs cannot be kept low. Furthermore, electroless plating must be performed in order to directly plate onto a ceramic substrate and this means that a great deal of time is required for the plated film to grow. In addition, the thickness of the plated film cannot be set larger than the film thickness achieved when thick film printing is employed. Thus, with a high frequency coil, for instance, it is difficult to achieve a high coil Q value.
The thin film technologies are extremely effective for achieving very fine electrode patterns, for achieving a high degree of accuracy in the electrical characteristics and for achieving miniaturization of parts. However, warpage resulting from firing normally occurs in a ceramic substrate. Because of this, it is difficult to adopt ceramic substrates in semiconductor manufacturing technology. For instance, in the photolithography process, the distance between the photo-mask and the wafer constituted of a ceramic substrate becomes inconsistent at different points on the surface of the wafer due to the warpage of the wafer, resulting in a reduced pattern accuracy.